MIDDLE CHALK—MICROSCOPIC STRUCTURE. 
529 
The amounts of chlorine would be equal to 2*023 and '14 
respectively of chloride of sodium, and its presence is doubtless 
due to the saturation of the rock by the sea-spray. 
The following is an analysis by Mr. Prangley of a sample of hard 
chalk used as a flux in smelting the brown iron-ore of the Corallian 
Beds at Westbury, Wiltshire. The specimen formed part of a 
series exhibited by the Westbury Iron Company in the Inter¬ 
national Exhibition of 1862, and afterwards presented to the 
Museum of Practical Geology. 
Inquiry at the works elicited the information that the chalk 
was obtained from the large quarry on Westbury Hill; the lower 
part of the chalk there exposed is hard, and belongs to the zone of 
Rhynchonella Cuvieri, not far above the Melbourn Bock (see p. 454). 
Water - 
_ 
. 260 
Organic matter 
1.865 
Silica - 
1.110 
Carbonate of iron and traces 
of alumina 
2.677 
Phosphate of lime 
- 
.523 
Carbonate of lime 
... . 
- 93.104 
Carbonate of magnesia 
- 
» - .188 
Sulphate of lime 
- 
. 132 
Chloride of sodium - 
_ 
trace 
Loss - 
.141 
100.000 
Chalk of the Terebratulina Zone. 
Very few analyses of this part of the chalk appear to have been 
made, probably because it is not very frequently exposed in quarries, 
and has been dug chiefly for burning into lime and making into 
whitening. It is a very pure chalk, and, so far as it has been 
examined, it never contains less than 96 per cent, of carbonates, 
and sometimes has as much as 99 per cent, (see table on p. 521). 
The analysis of a Chalk which was probably taken from this 
zone near Farnham in Surrey has already been given (p. 526). 
Mr. R. A. Berry has made an analysis of a sample from the 
“ upper part of the Middle Chalk ” at Royston, and has kindly 
placed the result at our disposal, this is as follows :— 
Calcium carbonate ------ - 85.93 
„ phosphate 
.80 
Excess of lime - 
- 5.85 
Magnesium carbonate 
- 3.63 
Oxide of iron and alumina 
.21 
Manganese dioxide 
.15 
Colloid silica 
1.48 
Clay and quartz 
- 1.20 
Loss on ignition 
.17 
Loss in analysis 
.58 
100.00 
